New AlGaAs buried heterostructure (BH) formation processes based on low temperature (T < 600 degree(s)C) in-situ mesa melt-etching and liquid phase epitaxial (LPE) regrowth have been studied and applied to the fabrication of BH laser diodes. It was shown that the BH formation process on AlxGa1-xAs laser structures with x equals 0.2...0.5 of cladding layers and masking stripes oriented along [011] or [011] on (100) planes depends on melt-etching rate anisotropy, while in a similar process for x > 0.6, or masking stripe orientations other than [011] or [011] the most important factors is the melt-etching material selectivity. Difficult AlGaAs nucleation on A-type planes is a distinct feature of LPE regrowth at temperatures lower than 600 degree(s)C. Low temperature melt-etching and regrowth produced single mode AlGaAs BH laser diodes emitting at 800 nm with the maximum optical power of 120 mW at 106 mA.